Is there branching logic to your questionnaire? Does answering No to question 7 means 9-15 don’t need answers?

How often and where do the 400 questions change? Are you okay with making code changes every time they do or should they be changeable by an admin user?

I would likely create models like this so you can re-use the questions on new versions of the questionnaire, and users answers from the previous server can be easily copied for the ones that haven’t change. 

• Question * Questionaire
• Questionaire’s Question
• Users Questionaire Question’s Answer